Generally, as a prerequisite to operating a certain device by an electric power, it is necessary to perform power feeding within a given operating voltage range which is determined by characteristics of the device. Further, an energy storage-type power supply system usable, for example, as an uninterruptible power supply (UPS) for emergency purposes, is required to have a sufficiently high capacitance so as to enable a prolonged operation in a situation where no electric power is externally fed thereto. Considering the above needs, in cases where an energy storage cell composed of a capacitor, an electric double-layer capacitor, a lithium ion capacitor, a secondary battery or the like is used as a power supply, as an effective way of obtaining a desired output voltage and capacitance, a power supply system may be constructed by connecting a plurality of energy storage cells in series and in parallel to form an energy storage module. In the power unit, in order to avoid problems, such as a problem that a high voltage is applied to only a part of the energy storage cells to cause rapid degradation thereof and a problem that only a part of the energy storage cells contribute to electric discharge without effectively utilization of energy stored in the remaining energy storage cells, it is also effective to operate the energy storage module while performing mutual charging and discharging between the energy storage cells to equalize voltages of the energy storage cells.
Such an energy storage module adapted to be operated while equalizing the voltages of the energy storage cells has been proposed by the inventors of the present invention (the following Patent Document 1: the invention described in the Patent Document 1 will hereinafter be referred to as “the previous patented invention”).
One example of an energy storage module according to the previous patented invention is illustrated in FIG. 1. The energy storage module illustrated in FIG. 1 comprises: a first series circuit constructed by connecting three energy storage cells B1B, B2B, B3B in series; a second series circuit constructed by connecting two energy storage cells B1A, B1A in series; a third series circuit constructed by connecting two energy storage cells B2C, B3C in series; a first switch group consisting of six switches Sa1 to Sa6; and a second switch group consisting of six switches Sb1 to Sb6. During operation, under control of a driver (FIG. 1), switching is performed between a first connection state as illustrated in FIG. 2 which is to be attained when each of the switches Sa1 to Sa6 making up the first switch group is turned on (i.e., set to an ON state), and a second connection state as illustrated in FIG. 3 which is to be attained when each of the switches Sb1 to Sb6 making up the second switch group is turned on (i.e., set to an ON state). Thus, each of the energy storage cells making up the energy storage module is subjected to mutual charging and discharging with respect to all of the remaining energy storage cells other than itself, directly or indirectly (through other energy storage cells), so that a variation between respective ones of the voltages of the energy storage cells will move toward being eliminated. In the energy storage module illustrated in FIG. 1, in order to allow a composite capacitance of parallel-connected energy storage cells (which are connected to other parallel-connected energy storage cells in series) to be kept constant in both of the first and second connection states, a ratio of a capacitance of each of the energy storage cells B1A, B2A, B2B, B2C, B3C to a capacitance of each of the energy storage cells B1B, B3B is set to 1:2. When the energy storage module is constructed by selecting the capacitance ratio in the above manner, a variation in voltage across each of rows of the parallel-connected energy storage cells will move toward being eliminated in each of the first and second connection states, so that it becomes possible to more quickly equalize the voltages of the energy storage cells.
In the previous patented invention, it is proposed to use a semiconductor switch as the switch constituting the first and second switch groups. However, neither a specific semiconductor switch to be used, nor a specific circuit configuration of a driver circuit for driving the semiconductor switch, is presented. Considering that, in the energy storage module according to the previous patented invention, a direction of current which promotes the mutual charging and discharging depends on a magnitude relationship between moment-to-moment voltages of the parallel-connected energy storage cells (i.e., in a design phase, the current direction cannot be determined to be one direction), each of the switches in an ON state needs to avoid blocking a current from flowing bidirectionally between the energy storage cells therethrough. Further, in order to prevent the mutual charging and discharging between the energy storage cells from occurring through the switches in an OFF state, each of the switches in the OFF state needs to bidirectionally block a current from flowing between the energy storage cells therethrough. In a design task for the energy storage module according to the previous patented invention, a problem will arise as to how each of the switch groups is constructed to meet the above needs.
The switch groups capable of meeting the above needs may be constructed by using a bidirectional switch as each of the switches making up the switch groups (i.e., a switch adapted, in its ON state, to avoid any blocking of bidirectional currents, and, in its OFF state, to bidirectionally block a current). One example of such a bidirectional switch is disclosed in the following Non-Patent Document 1.
A structure of a bidirectional switch disclosed in the Non-Patent Document 1 is illustrated in FIG. 4. The switch 100 illustrated in FIG. 4 comprises two MOSFETs in each of which a source electrode is connected to a base electrode, wherein the MOSFETs are integrated into one switch by connecting the source electrodes together and connecting respective gate electrodes of the MOSFETs together. In the switch 100, upon applying a gate voltage, each of the MOSFETs is turned on to enable a current to bidirectionally flow therethrough (considering that a parasitic diode which allows a current to flow from the source electrode to a drain electrode is formed in each of the MOSFETs, a conceptual pathway of a current which flows when each of the MOSFETs is turned on, is indicated by one of the two arrowed dotted lines in FIG. 5). On the other hand, when each of the MOSFETs is turned off due to no application of the gate voltage, a current flow in a direction indicated by each of the two arrowed dotted lines in FIG. 6 is blocked by the action of the parasitic diode formed in one of the MOSFETs.
The Non-Patent Document 1 also discloses a driver circuit for driving the switch 100 (FIG. 7). The driver circuit 200 illustrated in FIG. 7 comprises two photovoltaic couplers 201, 202, and two photo couplers 203, 204. The switch 100 is driven in such a manner that a switch ΦOi is turned on to allow a driving voltage from the photovoltaic couplers 201, 202 to be applied to the gate electrode of the switch 100 so as to turn on the switch 100, and then the switch ΦOi and a switch ΦOi are turned off and turned on, respectively, to short-circuit between the gate electrode and the source electrode of the switch 100 to release electric charges stored between the gate and source electrodes so as to turn off the switch 100.
The energy storage module according to the previous patented invention may be operated by employing the switch 100 disclosed in the Non-Patent Document 1 as each of the switches in the energy storage module illustrated in FIG. 1, and using the driver circuit 200 to drive these switches. However, the above switch is structurally complicated and uneconomical, in that the number of MOSFETs is required to be twice the number of the switches. Moreover, the driver circuit 200 requires two photovoltaic couplers and two photo couplers. In view of circuit simplification and cost reduction, it is desirable to drive each switch by a driver circuit with less number of elements.